1. Field of the Invention
The present invention relates to a current source circuit and a differential amplifier, and more particularly to a current source circuit realizing increase of an output resistance and a differential amplifier using this current source circuit.
2. Description of the Background Art
A current source providing a high output resistance (rds) is ideally suited in a semiconductor integrated circuit. However, a recent trend for downsizing process makes a shorter channel length of an MOS transistor. The output resistance of a current source including an MOS transistor is reduced accordingly by short channel effect.
In a differential amplifier using such a current source, the output resistance of the differential amplifier is reduced when viewed from a differential output terminal providing a differential signal. The differential amplifier provides a gain that is in proportion to the output resistance of the differential amplifier when viewed from the differential output terminal. Thus the reduced output resistance of the differential amplifier results in reduced gain of the differential amplifier.
The document named below tries to increase the gain of a differential amplifier using cascode connection:
“Design of Analog CMOS Integrated Circuits”, Behzad Razavi, MacGraw-Hil Series in Electrical and Computer Engineering, pp. 296-299 (FIGS. 9.6(b) and 9.8(b)).
However, the differential amplifier using cascode connection fails to prevent the reduction of an output resistance caused by short channel effect, thus making it difficult to increase the gain of the differential amplifier.